A conventional MOS transistor generally includes a semiconductor substrate, such as silicon, having a source, a drain, and a channel positioned between the source and drain. A gate stack composed of a conductive material (a gate conductor), an oxide layer (a gate oxide), and sidewall spacers, is typically located above the channel. The gate oxide is typically located directly above the channel, while the gate conductor, generally comprised of polycrystalline silicon (polysilicon) material, is located above the gate oxide. The sidewall spacers protect the sidewalls of the gate conductor.
Generally, for a given electric field across the channel of a MOS transistor, the amount of current that flows through the channel is directly proportional to a mobility of carriers in the channel. Thus the higher the mobility of the carriers in the channel, the more current can flow and the faster a circuit can perform when using high mobility MOS transistors. One mechanism to increase the mobility of the carriers in the channel of an MOS transistor is to produce a mechanical stress or strain in the channel.
A compressive strained channel typically provides hole mobility enhancement, which is particularly beneficial for PMOS devices, whereas a tensile strained channel typically provides electron mobility enhancement, which is particularly beneficial for NMOS devices. An exemplary method of introducing tensile strain in a channel region is to cover transistor devices with a capped poly layer comprised of a selected material that, when annealed, generates the tensile strain in the channel regions of NMOS devices. However, the capped poly layer can degrade hole mobility in channel regions of PMOS devices and cause performance of PMOS transistor devices to degrade.